function altRows(id){
if(document.getElementsByTagName){  

var table = document.getElementById(id);  
var rows = table.getElementsByTagName("tr"); 
 
for(i = 0; i < rows.length; i++){          
if(i % 2 == 0){
rows[i].className = "evenrowcolor";
}else{
rows[i].className = "oddrowcolor";
}      
}
}
}

window.οnlοad=function(){
altRows('alternatecolor');
}